Poor seedling performance and reduced seed emergence are often ascribed to known pathogens that cause low seedling recruitment and poor seed emergence in forest nurseries and regeneration plantings. On the other hand, foliar endophytes are often overlooked as a source of poor emergence or tree seedling disease. Here, we show that an endophytic fungus common to the foliar microbiome of Pinus ponderosa acts as a cryptic pathogen in delaying emergence. In a series of experiments, we inoculated seed of P. ponderosa with a suspension of Sydowia polyspora 12 h prior to sowing. S. polyspora reduced seed emergence of its host, P. ponderosa, by as much as 30%. A tetrazolium chloride viability assay showed that S. polyspora reduces emergence by preventing germination; seed remained viable. In sum, pathogens affecting tree seed emergence and seedling recruitment may be endophytic as well as in seed and soil and deserve greater attention in studies of natural regeneration.

Wildland fire is a major producer of aerosols from combustion of vegetation and soils, but little is known about the abundance and composition of smoke’s biological content. Bioaerosols, or aerosols derived from biological sources, may be a significant component of the aerosol load vectored in wildland fire smoke. If bioaerosols are injected into the upper troposphere via high-intensity wildland fires and transported across continents, there may be consequences for the ecosystems they reach. Such transport would also alter the concept of a wildfire’s perimeter and the disturbance domain of its impact. Recent research has revealed that viable microorganisms are directly aerosolized during biomass combustion, but sampling systems and methodology for quantifying this phenomenon are poorly developed. Using a series of prescribed fires in frequently burned forest ecosystems, we report the results of employing a small rotary-wing unmanned aircraft system (UAS) to concurrently sample aerosolized bacteria and fungi, particulate matter, and micrometeorology in smoke plumes versus background conditions. Airborne impaction-based bioaerosol sampling indicated that microbial composition differed between background air and smoke, with seven unique organisms in smoke vs. three in background air. The air temperature was negatively correlated with the number of fungal colony-forming units detected. Our results demonstrate the utility of a UAS-based sampling platform for active sampling of viable aerosolized microbes in smoke arising from wildland fires. This methodology can be extended to sample viable microbes in a wide variety of emissions sampling pursuits, especially those in hazardous and inaccessible environments.

Fusarium species coexist as toxigenic, systemic pathogens in sweet corn seed production in southwestern Idaho, USA. We hypothesized that fungal antagonists of seedborne Fusarium would differentially alter production of Fusarium mycotoxins directly and/or systemically. We challenged the Fusarium complex by in vitro antagonism trials and in situ silk and seed inoculations with fungal antagonists. Fungal antagonists reduced growth and sporulation of Fusarium species in vitro from 40.5% to as much as 100%. Pichia membranifaciens and Penicillium griseolum reduced fumonisin production by F. verticillioides by 73% and 49%, respectively, while P. membranifaciens and a novel Penicillium sp. (WPT) reduced fumonisins by F. proliferatum 56% and 78%, respectively. In situ, pre-planting inoculation of seeds with Penicillium WPT systemically increased fumonisins in the resulting crop. Morchella snyderi applied to silks of an F1 cross systemically reduced deoxynivalenol by 47% in mature seeds of the F2. Antagonists failed to suppress Fusarium in mature kernels following silk inoculations, although the ratio of F. verticillioides to total Fusarium double with some inoculants. Fusarium mycotoxin concentrations in sweet corn seed change systemically, as well as locally, in response to the presence of fungal antagonists, although in Fusarium presence in situ was not changed.

Pseudomonas syringae is a ubiquitous plant pathogen, infecting both woody and herbaceous plants and resulting in devastating agricultural crop losses. Characterized by a remarkable specificity for plant hosts, P. syringae pathovars utilize a number of virulence factors including the type III secretion system and effector proteins to elicit disease in a particular host species. Here, two Pseudomonas syringae strains were isolated from diseased Populustrichocarpa seeds. The pathovars were capable of inhibiting poplar seed germination and were selective for the Populus genus. Sequencing of the newly described organisms revealed similarity to phylogroup II pathogens and genomic regions associated with woody host-associated plant pathogens, as well as genes for specific virulence factors. The host response to infection, as revealed through metabolomics, is the induction of the stress response through the accumulation of higher-order salicylates. Combined with necrosis on leaf surfaces, the plant appears to quickly respond by isolating infected tissues and mounting an anti-inflammatory defense. This study improves our understanding of the initial host response to epiphytic pathogens in Populus and provides a new model system for studying the effects of a bacterial pathogen on a woody host plant in which both organisms are fully genetically sequenced.

Plants harbor a diverse community of microbes, whose interactions with their host and each other can influence plant health and fitness. While microbiota in plant vegetative tissues has been extensively studied, less is known about members of the seed microbiota. We used culture-based surveys to identify bacteria and fungi found in the seeds of the model tree, Populus trichocarpa, collected from different sites. We found that individual P. trichocarpa seeds typically contained zero or one microbe, with common taxa including species of Cladosporium, Aureobasidium, Diaporthe, Alternaria, and Pseudomonas, a bacterium. Pseudomonas isolates were associated with seed mortality and were negatively associated with the occurrence of fungal isolates within Epicoccum, Alternaria, and Aureobasidium from the same seed. Next, we conducted an inoculation experiment with one of the isolated seed microbes, Pseudomonas syringae pv. syringae, and found that it reduced seed germination and increased seedling mortality for P. trichocarpa. Our findings highlight common fungi and bacteria in the seeds of P. trichocarpa, prompting further study of their functional consequences. Moreover, our study confirms that P. syringae pv. syringae is a seed pathogen of P. trichocarpa and is the first report that P. syringae pv. syringae is a lethal seedling pathogen of P. trichocarpa, allowing for future work on the pathogenicity of this bacterium in seedlings and potential antagonism with other seed microbes.